Highly responsive and selective ozone sensor based on Ga doped ZnS–ZnO composite sprayed films

Ozone detection is currently the subject of wide scientific and technological research, motivated by its harmful impact on human safety, environment and health. With the aim of searching for new highly sensitive materials for ozone detection, Ga-doped ZnS and ZnS–ZnO films were deposited by a spray pyrolysis technique. The obtained films were annealed at 400 °C for two hours. The ozone sensing properties were investigated by measuring the sensor resistance for several ozone concentrations ranging from 30 to 120 ppb. The sensor response reveals a dependence on the gallium concentration. The best response was obtained with 4% doping gallium. The sensitivity is 4.5 ppb−1 at 260 °C and the response to 30 ppb ozone is 150. Moreover, the sensor shows high performance such as good selectivity and fast rapidity.


Introduction
In recent years, zinc sulde (ZnS) and zinc oxide (ZnO) have garnered signicant interest as highly promising II-VI semiconductor compounds with excellent characteristics.They exhibit a range of properties that contribute to their potential for fabricating photonic, optical, and electronic devices such as solar cells, 1,2 window layers 3,4 and photocatalysis. 5With a wide bandgap ranging from 3.54 to 3.91 eV and from 3.37 to 3.72 eV for ZnS and ZnO respectively, heterostructures of these compounds are candidates for ultraviolet lasers and detectors. 6,7In the literature, ZnS/ZnO compounds have been reported to be fabricated by different methods, such as: ZnS/ZnO heterostructures synthesized via a simple thermal evaporation method, 6,8 ZnS-ZnO nanocomposites through the hydrothermal route, 9 coaxial nanowires and hierarchical nanowires by chemical bath deposition and chemical etching processes, respectively, 10 and ZnS/ZnO lms via pulsed laser deposition. 11n another study, 12 ZnS/ZnO:Mn layers were prepared by thermal suldation of ZnO:Mn lms deposited on a Si substrate with an RF magnetron sputtering technique.In addition, Deepa et al. 13 have deposited zinc sulde thin lms by ultrasonic spray pyrolysis.Obtained lms have traces of the ZnO phase together with the dominant ZnS phase.Among these methods, spray pyrolysis is an interesting deposition technique for preparing ZnS/ZnO thin lms.Indeed, it is a simple and an inexpensive technique to obtain optically smooth, uniform and homogeneous layers.
Recently, researchers have shown that ZnS/ZnO could be a good candidate for gas detection.Tsai et al. 14 reported that ZnO/ ZnS core-shell structures hydrothermally grown on SiO 2 substrates show promise for future hydrogen sensing applications.More recently, 15 they demonstrated that CO gas sensing measurements revealed that incorporation of a ZnS shell on ZnO nanorods can increase gas sensing capability.In another work, Ding et al. 16 reported that ZnO nanowires were modied with an optimal amount of ZnS to form nanostructured heterojunctions for high-performance H2S gas sensing.Furthermore, Park et al. 17 reported that ZnS-core/ZnO-shell nanowires demonstrate substantial improvement in the response to NO 2 gas by UV irradiation.As ozone (O 3 ) is a derivative gas from NO 2 , to the best of our knowledge, there is no report on the characterization of ZnS/ ZnO in ozone sensing application.This prompts us to investigate ZnS/ZnO within the framework of this goal in this report.Ozone plays a vital role in supporting life as it acts as a primary UV protective layer in the stratosphere.It is a signicant environmental pollutant, known to cause detrimental effects on human health even at low concentrations.Exposure to tropospheric ozone can lead to irritation of the eyes, nose and throat, as well as respiratory issues such as coughing and headaches. 18,19To ensure the well-being of the public, the World Health Organization (WHO) has established guidelines limiting exposure to ozone at a maximum average concentration of 100 mg m −3 (50 parts per billion, ppb) for 8 hours period. 20Additionally, for the protection of vegetation, the European directive employs an accumulated ozone over the threshold of 80 mg m −3 (40 ppb, designed as AOT40).Several materials have already been studied for ozone detection such as Co 3 O 4 , 21 ZnO/SnO 2 , 22 In 2 O 3 , 23 WO 3 (ref.24) and CuAlO 2 . 25Generally, the selectivity of the sensors can be adjusted over a wide range by varying several aspects such as the crystal structure and morphology, the dopants used, 26 the contact geometries, 27 the operating temperature or the operation mode. 24n this paper, we demonstrate that ZnS-ZnO material is a good ozone sensor.Gallium doping improves its performances at high temperature (260 °C) and leads also to ozone detection at lower temperatures down to 120 °C.The sensing properties of the prepared lms were studied using DC electrical characterization.

Experimental
ZnS-ZnO lms were grown on Si/SiO 2 substrates using the spray pyrolysis technique.This method is a simple and cost effectiveway to deposit large area thin lms at moderate temperatures (100-500 °C).The lms were prepared by spraying an aqueous solution onto the substrates.The solution contained zinc chloride (ZnCl 2 ) and thiourea [SC(NH 2 ) 2 ], as sources of Zn 2+ and S 2− ions, respectively, with an initial Zn:S molar ratio of 1. Distilled water was used as a solvent.To achieve various levels of gallium (Ga) doping, gallium sulphate Ga 2 (SO 4 ) 3 with three different concentrations (0%, 2% and 4% atomic concentration) was added to the starting solution.The substrates were placed on a hotplate and heated gradually from ambient temperature until reaching the desired deposition temperature of 350 °C.This gradual heating helps prevent thermal shock.The spray duration was kept at 3 minutes.The distance between the spray nozzle and the substrate was xed at 25 cm to ensure proper coverage of the substrate surfaces.During the deposition, the solution and nitrogen carrier gas ow rates were maintained at 2 ml min −1 and 10 l min −1 , respectively.The formation of ZnS is given by the following equation: Aer deposition, thin lms were annealed at 400 °C in an electric furnace under atmospheric air.The purpose of this annealing was to improve crystallization and stability of the sensitive lms.The heating rate was xed at 10 °C min −1 to reach 400 °C, which was maintained for 2 h.Then, the cooling was carried out in atmospheric air within the furnace.To measure the electrical resistance of the sensors, transducer platforms were used.These platforms had two interdigitated Pt/ Ti electrodes sputtered on a Si/SiO 2 substrate, as shown in Fig. 1a.The interdigitated electrodes consisted of thirty rectangular platinum ngers on a 4 × 4 mm 2 substrate area, with 50 mm gap and nger width.
The microstructure of the deposited lms was analyzed by Xray diffraction, using a Bruker D8 Advance diffractometer, with a monochromatic Cu-Ka radiation (l = 1.5406Å).The angle 2q was varied from 20°to 70°during the measurements.The morphological characterization was performed by scanning electron microscopy (SEM), using a Zeiss FE-SEM ULTRA plus microscope with an attached electron dispersive spectrometer (EDS) for chemical composition analysis.
The ozone sensing properties of the samples were investigated using the experimental set-up shown in Fig. 1b.To produce ozone gas, a UV pen ray lamp (UVP/185 nm stable ozone generator) was used.Dry air was led through a quartz tube.When illuminated by a UV lamp, some of the oxygen molecules were transformed into ozone.The UV illumination was modied by moving the shutter around the lamp.Several ozone concentrations in the range of 30 ppb to 120 ppb were obtained while maintaining the dry air ow at a xed rate of 0.5 l min −1 .Calibration of ozone concentrations was ensured by an Seres 2000G ozone analyzer.The sensors were exposed to ozone for 3 minutes.The operating temperature was regulated by a heat source from 120 to 260 °C.Indeed, the sensor is brought to working temperature using a resistance covered with ceramic acting as a sample holder and controlled by a stabilized generator.The heating is adjusted manually and the heating rate is practically 10°min −1 .A Pt100 probe, xed in the vicinity of the sample and connected to an ohmmeter, is used to display the sensor temperature.The sensors were polarized at 1 V.The resistance measurements were performed using a Keithley 2450 source-meter.In order to calculate the response of the ZnS sensors, based on the change in resistance, we applied eqn ( 2) and ( 3) for oxidant and reducing gases, respectively: where R air over R gas is the ratio of the sensor resistance value in dry air and target gas, respectively.Sensitivity is dened by the following formula: where C gas is the concentration of the target gas.The slope of the tting line between the response and the concentration corresponds to the sensitivity S of the tested sensor.
3 Results and discussion  002) and (101).The other peaks, labeled as (102), (110), and (103) matching angles 47°, 57°and 63°, respectively, are attributed to the hexagonal ZnS phase (according to the JCPDS card no.036-1450).Ga ions were incorporated into the hexagonal structure lattice.This indicates that the gallium doping improves the crystal structure of the lms, resulting in an increase in the diffraction peak intensity.As consequence, these ndings suggest that the samples have polycrystalline character and can be identied as ZnO-ZnS composite, which is in agreement with the work of Ali et al. 9 Structural parameters such as crystallite size D, dislocation density d and microstrain 3 were estimated from the XRD proles for the preferred orientation (002).
Assuming spherical crystallites, their size can be estimated using Debye-Scherrer's formula as follows: 26 where l is the X-ray wavelength (l = 1.5406Å), q is the Bragg angle and b is the full width at half maximum (FWHM).
The dislocation density was calculated using the formula: The microstrain was deduced from the following relation: The calculated values of D, d and 3 are summarized in Table 1.The crystallite size is approximately 18 nm.The increase in micro-stress is attributed to the doping.
3.1.2.Chemical composition.Quantitative elemental chemical composition of undoped and gallium-doped ZnS-ZnO was determined using energy dispersive spectroscopy (EDS).EDS spectra conrm the presence of Zn and S and showed the existence of the gallium dopant in the layer (Fig. 3).Additionally, the spectra revealed the presence of Pt, Si, and O, which are the constituents of the interdigital electrodes and the substrate.
3.1.3.Morphological characterization.Fig. 4 shows the topographic images of all the samples, revealing a well-covered surface layer.The undoped ZnS sample (Fig. 4a) exhibits a compact layer with a smooth surface.Additionally, the grains are very small, indicating the amorphous structure of the lm, which is in agreement with the XRD results.On the other hand, the image of the 2% Ga doped sample (Fig. 4b) shows the most porous layer, while the 4% Ga-doped sample (Fig. 4c) appears more compact and more granular.These results may be the consequence of a deviation from the stoichiometric composition in the starting solution.This deviation results in an incomplete reaction between the precursors, which has a very strong inuence on microstructure and surface morphology.Similar results have been previously observed on In 2 S 3 and CuInS 2 lms. 27,28.1.4.Optical characterization.The transmittance spectra (T) of the different lms are shown in Fig. 5a.It is clear that all the samples have high transmittance in the visible range,   possible to replace CdS in CIGS solar cells by these lms. 31In all transmittance spectra, there is an absence of interference fringes due to weak multiple reections at the interface.In addition, the absorbance spectra in the inset of Fig. 5a show that the absorbance is high for all lms in the UV-region (l < 350 nm), corresponding to the fundamental absorption.Moreover, at wavelengths greater than 350 nm (Vis-IR regions), which correspond to the transparence region, the absorbance has a low value.
The absorption spectra were used to calculate the optical band gap energies of the lms using Tauc expression: where A is constant and E g is the band gap energy.
We have studied the evolution of (ahn) 2 as a function of the photon energy hn for the different layers (Fig. 5b).The extrapolation of the linear zone of the obtained curve to the horizontal axis (ahn = 0) gives us a good estimate of the bandgap energy.
The evaluated E g of undoped sample is equal to 3.84 eV, which decreases slightly with doping to 3.74 eV.The bandgap energy of the undoped sample is larger than that reported for bulk ZnS.This is due to residual stress. 33This result is consistent with previous studies by Poornaprakash et al. (3.93-3.70 eV) 34 and Wei et al. (3.87-3.76eV). 352.Ozone sensing study 3.2.1.Ozone test and sensing mechanism.In order to test the sensitivity of the prepared samples to ozone, measurements of the resistance were carried out for ozone concentrations less than 120 ppb during an exposure time of 3 minutes at 260 °C.Fig. 6(a-c) shows the dynamic change in resistance during repetitive cycles of alternating exposure to ozone and dry air.
We can observe that the resistance of all lms increases during exposure to O 3 and decreases when dry air is reintroduced into the test chamber.This behavior is the result of adsorption-desorption phenomenon of an oxidizing gas on the surface of an n-type semiconductor. 36We also note that the baseline is stable, and all samples have good repeatability indicating the material's potential to detect O 3 gas.We suggest that the detection mechanism can be explained by dissociative adsorption alone for triatomic gas.Firstly, when ozone is added to the atmospheric gas, O 3 molecules adsorb on the surface sites of the sensitive layer according to the following equation: The trapping of electrons by ozone leads to a decrease of free electron concentration and as a consequence, resistance increases.
In the other side, during the introduction of dry air, O − is desorbed from the surface of the material into the gas phase in the form of ozone molecules as shown in the following equation: 3.2.2.Ozone sensing performances 3.2.2.1 Response.In order to monitor the effect of the working temperature on the ozone sensor response, the lms were heated to ve temperatures ranging from 120 °C to 260 °C.
Fig. 7a illustrates the resistance change of undoped ZnS sample over time for 120 ppb ozone at 260 °C.Below this temperature, the material exhibits high resistance and no signal can be recorded.It now states that the resistance is very high below a certain temperature and it cannot be measured by the Keithley nanoammeter.Fig. 7b and c illustrate the dynamic change in sensor resistance for 120 ppb of ozone at ve operating temperatures.We can note that the resistance base line diminishes by rising operating temperature, conrming the semiconductor behavior of the material.Fig. 7d shows that the response increases from 1.7 to 50 for ZnS-ZnO doped Ga2% and from 5.6 to 540 for ZnS-ZnO doped Ga4% by increasing the operating temperature from 120 to 260 °C.While, the undoped layer shows a weak response of 2.8 at an operating temperature of 260 °C.In fact, we think that as the temperature increases, the electron density of the ZnS-ZnO composite increases resulting of the electrons emission from donor levels matching sulfur  vacancies, oxygen vacancies and interstitial Zn to the conduction band.This increase in the concentration of free electrons promotes the chemisorption of O 3 molecules according to eqn (9).In addition, gallium (Ga) is located in group 13, so it therefore has 3 valence electrons, while the zinc has two only.Most importantly, the higher the doping level gets, the higher of the electron density in the ZnS-ZnO composite which further improves the ozone adsorption. 37As a result, the sensing response and layer conductivity can be enhanced by Ga doping.ZnS-ZnO:Ga4% is our best candidate to detect ppb level ozone gas.
Fig. 8(a-c) show the dynamic response of the sensors to the different ozone concentrations.These results indicate that there is a proportionality between the response of the sensors and the concentration of ozone.We observed that the response of the undoped layer increased from 1.5 to 2.8 by increasing the ozone concentration from 30 ppb to 120 ppb.For ZnS-ZnO:Ga2%, the response increased from 28 to 55.The best response was obtained with the ZnS-ZnO:Ga4% sample, which showed an increase in response from 140 to 540 by increasing the ozone concentration in the same range.Fig. 8d depicts the evolution of the sensor response as a function of ozone concentration.The response is linearly proportional to O 3 concentration.The sensitivity of the sensor is calculated from the slope of the tted line and is equal to 0.01 ppb −1 , 0.3 ppb −1 , and 4.2 ppb −1 corresponding to undoped ZnS, ZnS-ZnO:Ga2%, and ZnS-ZnO:Ga4%, respectively.Consequently, gallium doping improves both sensitivity and response and can be suitable for gas applications.
3.2.2.3 Rapidity.The sensor's performance can be evaluated in terms of its response and recovery times, which are important indicators of its rapidity.The response time is dened as the time necessary for the sensor to achieve 90% of the response reaching a saturate state, and the recovery time represents the time to return to 10% of the baseline aer exposure to ozone.
Fig. 9a illustrates the evolution of response and recovery times versus working temperature of a ZnS-ZnO:Ga4% sensor exposed to 120 ppb of ozone.It can be noted that the response time is in the order of 160 seconds and the recovery time decreases from 1860 s to 45 s by rising the operating temperature from 120 °C to 260 °C.In fact, when energy is supplied to a system, the temperature rises, which can accelerate the chemical reactions controlling gas adsorption and desorption at the semiconductor surface.With this, the activation energy for the recovery process can be calculated from the dependence of recovery time as a function of temperature, using the wellknown Arrhenius equation: where E a is the activation energy required by the sensor for the recovery process for ozone, k B is the Boltzmann constant and T is the absolute operating temperature of the sensor.Fig. 9b reveals that ln(s rec ) varies linearly with 1000/T, indicating that the estimated value of the activation energy for the recovery process is 0.5 eV, which is higher than the thermal energy at room temperature (E th = 0.025 eV).This result suggests that ozone desorption process is difficult and recovery time is high at low temperature.
3.2.2.4 Selectivity.The selectivity of a sensor refers to its ability to specically respond to a particular gas in the presence of other gases.To investigate the selectivity of the ZnS-ZnO:Ga4% sensor, the last was exposed to various volatile organic compounds (VOCs) at 500 ppm concentration and nitrogen dioxide (NO 2 ) at 120 ppb at a working temperature of 260 °C.Fig. 10a displays the current change versus time as the sensor undergoes alternating cycles of exposure to VOCs and dry air.The results of obtained response are illustrated by the histogram in Fig. 10b.We have also reported the response of the same sample for an ozone and a nitrogen dioxide concentration of 120 ppb.The results clearly demonstrate the high selectivity of the ZnS-ZnO:Ga4% sensor towards ozone in presence of VOCs and nitrogen dioxide.
A comparative analysis of the sensing properties of the developed ZnS-ZnO:Ga4% sensor with previously published ozone sensors is presented in Table 2. [38][39][40][41][42][43] The results highlight that ZnS-ZnO:Ga4% sensor outperforms other ozone sensors in terms of detection sensitivity.Specically, it shows promising potential for detecting ozone gas.

Conclusion
In this work, undoped and Ga-doped ZnS-ZnO thin lms were deposited on SiO 2 /Si substrates with inter-digitated platinum electrodes by spray pyrolysis technique at a temperature of 350 °C.The effect of Ga doping on structural, morphological, optical and ozone gas sensing properties has been reported.The ndings indicate the amorphous structure for the undoped lms while with Ga doping the lms present two hexagonal phases: ZnS and ZnO.Doping improves considerably crystallinity and the average crystallite size was evaluated to be 18 nm.In terms of surface morphology, the undoped sample was homogenous and smooth, but with doping, it became porous and granular.Moreover, the optical transmission was high in the visible region and reaches 90% in value.The gap energy decreased from 3.84 eV to 3.74 eV by doping.Finally, the ozone detection performances were evaluated.The best properties were found with ZnS-ZnO:Ga4% sensor.At 260 °C working temperature, the response to 30 ppb ozone is 150, the sensitivity is 4.5 ppb −1 , the response recovery times were found to be 160 s and 45 s respectively.This study suggests that the gallium-doped ZnS-ZnO composite deposited by spray pyrolysis pathway has the potential for highly sensitive and ppb level ozone detection.Nevertheless, these results are encouraging for deepening the study of sensing mechanism to improve performances of ZnS-ZnO:Ga promising sensor.

Fig. 1
Fig. 1 (a) Photo of platform for transducers and (b) ozone detection experimental set-up.

3. 1 .
Physical characterization 3.1.1.Structural analysis.The XRD patterns of the prepared samples are shown in Fig. 2. No characteristic XRD peak is observed for the undoped sample, revealing its amorphous structure, while those of the Ga doped exhibit narrower diffraction peaks, indicating an improved crystallinity.The observed diffraction pattern corresponds to the characteristic lattice planes of two phases: ZnS and ZnO.The three prominent peaks at Bragg angles 31°, 34°and 36°correspond to the hexagonal structure of ZnO (JCPDS card no.36-1451) and are labeled as (100), (

Fig. 2 X
Fig. 2 X-ray diffraction patterns thin films for different Ga concentrations.

Table 1
Structural parameters of Ga doped ZnS-ZnO thin films Sample D (nm) d (10 10 lines per cm 2 ) 3 © 2024 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 413-423 | 415 Paper RSC Advances ranging from 80 to 90%.The effect of doping is remarkable on the absorption front.The later moves towards the long wavelengths, which makes it possible to predict a decrease in the bandgap energy with doping.This result is in good agreement with those reported by Shu-wen et al., 29 who used Na-doped ZnS, and Derbali et al. who used Ni-doped ZnS. 30 Therefore, it is

Table 2
Sensing properties of the developed ZnS-ZnO:Ga4% sensor compared with previously published sensors